Method of controlling a wind turbine generator

ABSTRACT

A method for use in controlling a wind turbine generator of a wind power plant based on a condition of a power converter or a component forming part of a power converter in the wind power plant. The method comprises determining the initial condition of the power converter or a component forming part of a power converter and determining the evolution of the condition from the initial condition based on notional power reference values of the wind turbine generator. The method further comprises comparing the evolution of the condition to a predetermined threshold and determining, from the comparison, a time period by which the condition of the power converter or a component forming part of a power converter will substantially equal the predetermined threshold.

The present invention relates to a method for use in controlling ofpower converter in a wind turbine generator of a wind power plant.

BACKGROUND TO THE INVENTION

A wind turbine generator converts energy contained in wind intoelectrical power, which is typically delivered to a power grid. As thechangeable nature of wind entails an electrical power output of varyingcharacteristics, a power converter is also included to modify thevariable frequency electrical power produced by the generator into afixed frequency electrical power output that is more suitable fordelivery to a power grid.

To this end, a converter controller is provided for adjusting the powerconverter output according to ambient wind conditions. The convertorcontroller may also be used to manage the generator in view of safetyconsiderations and physical constraints in order to maintaincost-effective energy production by preventing component failure andminimising downtime.

It is against this background that the invention has been devised.

SUMMARY OF THE INVENTION

An aspect of the invention provides a method for use in controlling awind turbine generator of a wind power plant based on a condition of apower converter or a component forming part of a power converter in thewind power plant. The method comprises determining the initial conditionof the power converter or a component forming part of a power converterand determining the evolution of the condition from the initialcondition based on notional power reference values of the wind turbinegenerator. The method further comprises comparing the evolution of thecondition to a predetermined threshold and determining, from thecomparison, a time period by which the condition of the power converteror a component forming part of a power converter will substantiallyequal the predetermined threshold. Preferably, the condition of thepower converter is the temperature of the power converter or a componentforming part of a power converter.

Preferably, the method further comprises determining, based on the powerreference values, an active power limit and a reactive power limit.Preferably, the active power limit and the reactive power limit take theform of a P-Q chart.

Preferably, the method further comprises prioritising between activepower and reactive power when determining the active power limit and thereactive power limit.

Preferably, the method further comprises controlling the wind turbinegenerator at the notional power reference values for a duration notexceeding the time period.

Preferably, the initial temperature and the temperature evolution aredetermined as a non-linear function based on the power loss of the powerconverter or a component forming part of a power converter at currentoperating parameters and an ambient temperature.

Preferably, the power loss of the power converter or a component formingpart of a power converter is determined based on the voltage across thepower converter and the current through the power converter. Morepreferably, the power loss of the power converter is the product of thevoltage across the power converter and the current through the powerconverter. Each of these properties may be either directly measured orotherwise estimated.

Preferably, the method, when applied to a plurality of power converters,further comprises selecting, from a plurality of time periods, theshortest time period as the time period.

Preferably, the ambient temperature is a measurable temperature nearestto the power converter. That is, the ambient temperature is thetemperature of an area surrounding the power converter so far as it isreasonably practicable to measure. This may include, for example, theambient air temperature.

Preferably, the method further comprises comparing the initial conditionof the power converter to the predetermined threshold and modifying thepower reference values of the wind turbine generator if the conditionsubstantially equals or exceeds the predetermined threshold. Inparticular, the step of modifying the current power reference values ofthe wind turbine generator comprises derating the parameters.

Another aspect of the invention provides controller for a wind turbinegenerator comprising a data processing means and a memory module,wherein the memory module comprises a set of program code instructionswhich when executed by the data processing means implement a methodaccording to the first aspect of the invention.

Another aspect of the invention provides a computer program productdownloadable from a communication network and/or stored on a machinereadable medium comprising program code instructions for implementing amethod according to the first aspect of the invention.

It will be appreciated that preferred and/or optional features of thefirst aspect of the invention may be incorporated alone or inappropriate combination in the second aspect of the invention also.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the invention will now be described, byway of example only, with reference to the following drawings, in which:

FIG. 1 is a schematic diagram of a wind turbine generator that issuitable for use with embodiments of the invention;

FIG. 2 is a schematic diagram of an architecture of a full-scaleconverter based wind power plant that is suitable for use withembodiments of the invention;

FIG. 3 is a block diagram representation of a convertor controller ofthe convertor in FIG. 2;

FIG. 4 is a block diagram of a thermal capability manager of theconvertor controller of FIG. 3;

FIG. 5 is a block diagram of a component thermal model of the thermalcapability manager of FIG. 4;

FIG. 6 is a thermal model of the component thermal model of FIG. 5; and,

FIG. 7 is a representation of a typical P-Q chart used by the convertercontroller of FIG. 3;

FIG. 8 is a graph illustrating the evolution of the temperature of acomponent using the component thermal model of FIG. 5;

FIG. 9 is a schematic block diagram of an architecture of a DFIGarrangement that is suitable for use with embodiments of the invention.

In the drawings, like parts are denoted by like reference signs.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

To provide context for the invention, FIG. 1 shows an individual windturbine generator 1 of a kind that may be controlled according toembodiments of the invention. It should be appreciated that the windturbine generator 1 of FIG. 1 is referred to here by way of exampleonly, and it would be possible to implement embodiments of the inventioninto many different types of wind turbine systems.

The wind turbine generator 1 shown is a three-bladed upwindhorizontal-axis wind turbine (HAWT), which is the most common type ofturbine in use. The wind turbine generator 1 comprises a turbine rotor 2having three blades 3, the rotor 2 being supported at the front of anacelle 4 in the usual way. It is noted that although three blades iscommon, different numbers of blades may be used in alternativeembodiments. The nacelle 4 is in turn mounted at the top of a supporttower 5, which is secured to a foundation (not shown) that is embeddedin the ground.

The nacelle 4 contains a generator (not shown in FIG. 1) that is drivenby the rotor 2 to produce electrical energy. Thus, the wind turbinegenerator 1 is able to generate power from a flow of wind passingthrough the swept area of the rotor 2 causing the rotation of the blades3.

With reference now to FIG. 2, an example of a wind power plant 12 towhich methods according to embodiments of the invention may be appliedis shown. The example shown is representative only and the skilledreader will appreciate that the methods described below may beapplicable to many different configurations. For example, although theexample shown in FIG. 2 is based on a full-scale converter architecture,in practice the invention may be used with other types of converter andin general terms the invention is suitable for use with all topologies,such as DFIG arrangements.

Moreover, the power converter or components forming part of a powerconverter of the wind power plant 12 are conventional and as suchfamiliar to the skilled reader, and so will only be described inoverview.

The wind power plant 12 shown in FIG. 2 includes a single wind turbinegenerator 1 such as that shown in FIG. 1, but in practice further windturbine generators may be included.

As already noted, the wind turbine generator 1 comprises an electricalgenerator 20 that is driven by a rotor (not shown in FIG. 2) to produceelectrical power. The wind turbine generator 1 includes a low voltagelink 14 defined by a bundle of low voltage lines 16 terminating at acoupling transformer 18, which acts as a terminal that connects the windturbine generator 1 to a grid transmission line that in turn connects toa power grid. Electrical power produced by the wind turbine generator 1is delivered to the grid through the coupling transformer 18.

The power produced in the electrical generator 20 is three-phase AC, butis not in a form suitable for delivery to the grid, in particularbecause it is typically not at the correct frequency or phase angle.Accordingly, the wind turbine generator 1 includes a power converter 22and a filter 24 disposed between the electrical generator 20 and thecoupling transformer 18 to process the electrical generator 20 outputinto a suitable waveform having the same frequency as the grid and theappropriate phase angle.

The power converter 22 provides AC to AC conversion by feedingelectrical current through an AC-DC converter 26 followed by a DC-ACconverter 28 in series. The AC-DC converter 26 is connected to the DC-ACconverter 28 by a conventional DC link 30, which includes a switch 31 inseries with a resistor 32 to act as a dump load to enable excess energyto be discharged, and a capacitor 34 providing smoothing for the DCoutput.

Any suitable power converter 22 may be used. In this embodiment, theAC-DC and DC-AC parts of the power converter 22 are defined byrespective bridges of switching devices (not shown), for example in theconfiguration of a conventional two level back-to-back converter.Suitable switching devices for this purpose include integrated gatebipolar transistors (IGBTs) or metal-oxide-semiconductor field-effecttransistors (MOSFETs). The switching devices are typically operatedusing pulse-width modulated drive signals.

The smoothed DC output of the AC-DC converter 26 is received as a DCinput by the DC-AC converter 28 and creates a three-phase AC output fordelivery to the coupling transformer 18.

As noted above, in a full-scale architecture the DC-AC converter 28 isconfigured to provide a level of control over the characteristics of theAC power produced, for example to increase the relative reactive powerin dependence on grid demand. Noting that the magnitude, angle andfrequency of the output is dictated by grid requirements, and that thevoltage is set at a constant level in accordance with the specificationsof the low voltage link 14, in practice only the current of the ACoutput is controlled, and a converter controller 36 is provided for thispurpose. The converter controller 36 forms part of an overall controlsystem that controls operation of the wind power plant 12, and isdescribed in more detail later with reference to FIG. 3.

The control system acts based on sample data obtained by a samplingsystem that probes the wind turbine generator 1 at various stages tosample electrical signals that are indicative of current and/or voltage,for example. In particular, as is typical for a full-scale architecture,the sampling system gathers raw data relating to the current and voltageof the outputs from the stator 23 of the generator 20, and from thepower converter 22 on the grid side. This raw data is processed intosample data, which is then passed to the converter controller 36, forexample. The converter controller 36 uses the sample data to determineoperating parameters for the wind turbine generator 1. For example, theduty cycle of the control signals for generator-side IGBTs of the powerconverter 22 may be determined, at least in part, based on theinstantaneous properties of the generated power supplied by theelectrical generator 20.

The AC output leaves the power converter 22 through the three powerlines 16, one carrying each phase, which together define the low voltagelink 14. The low voltage link 14 includes the filter 24, which in thisembodiment comprises a respective inductor 38 with a respective shuntedfilter capacitor 40 for each of the three power lines 16, to providelow-pass filtering for removing switching harmonics from the ACwaveform.

The three power lines 16 may also each include a respective circuitbreaker (not shown) for managing faults within the wind power plant 12.

As noted above, the low voltage link 14 terminates at the couplingtransformer 18, which provides a required step-up in voltage. A highvoltage output from the coupling transformer 18 defines a wind turbinegenerator terminal 42, which acts as a point of common coupling for thewind power plant 12.

The low voltage link 14 also includes three branches, one for eachphase, that define auxiliary power lines 44 that divert some of thepower that is output from the filter 24 for powering auxiliary systemsof the wind power plant 12 such as, for example, yaw, pitch and coolingsystems.

As some of the power that is output from the power converter 22, orP_(CONV), is diverted to provide power for auxiliary systems, orF_(AUX), it follows that P_(CONV) is greater than the power that isdelivered to the grid, or ‘line power’ P_(L).

The converter controller 36 may be configured to prioritise the variouspower references that it receives and correspondingly adjust the totalpower reference according to which the power converter 22 is controlled.

For example, if the total demanded power, namely the total grid demand,F_(L), combined with the total demand for powering auxiliary systems,F_(AUX), is greater than the total power that the wind power plant 12 iscapable of producing with respect to ambient temperature and windconditions, P_(CONV), demand will not be met somewhere in the system. Inthis situation, the converter controller 36 may control the powerconverter 22 in accordance with a list of prioritised power referencesto ensure that the total grid demand is met. Alternatively, the totalgrid demand may be sacrificed in favour of the total demand for poweringauxiliary systems.

As FIG. 3 shows, the converter controller 36 of this embodimentcomprises an active power controller 46, a reactive power controller 48,a software block defining a power management module 50 and a thermalcapability manager 49. The skilled reader will appreciate that inpractice the converter controller 36 may include various other controlmodules, but for the purposes of the present disclosure only those thatrelate to power control are referred to.

The active power controller 46 and the reactive power controller 48operate in tandem to interface to current controllers (not shown), whichissue drive signals to the switching devices of the power converter 22to control the active and reactive components of its AC output based onsignals received from the active and reactive power controllers 46, 48.The active power controller 46 is configured to receive an active powerreference from the power management module 50, and the reactive powermanager 48 is configured to receive a reactive power reference from thepower management module 50.

The power management module 50 provides a suite of functions that enablethe processing and optimisation of power references that arise withinthe wind power plant 12, and those received from external sources suchas a transmission system operator responsible for the grid, a powerplant controller responsible for multiple wind turbine generators withina single wind power plant, or a turbine controller, for example.

The power management module 50 is modularised, in that it comprises aset of discrete modules that each provide a specific function. In thisembodiment, those modules are implemented as individual software blockswithin a common processing unit, but in other arrangements dedicatedhardware modules could be used.

The modularised arrangement enhances integration with the convertercontroller 36, in particular because it enables individual functions tobe developed and upgraded without impacting other functions. Moreover, aclearly defined hierarchy between the different functions can becreated, thus improving interaction between the functions and thereforeimproving the efficiency of the converter controller 36.

More specifically, in this embodiment the power management module 50includes a power reference manager 52, a power capability manager 54,and a degrade mode manger 56. These modules are ordered according to ahierarchy in which the degrade mode manager 56 provides inputs to thepower capability manager 54, which in turn provides inputs to the powerreference manager 52, which then transmits an active power reference anda reactive power reference to the active power controller 46 and thereactive power controller 48, respectively.

The degrade mode manager 56 is arranged to degrade, or de-rate, thepower generating capability of the generator 20 based on instantaneousoperating parameters. For example, the generating capability of thegenerator 20 may be degraded if the temperature of a coolant system ofthe wind turbine generator 1 is higher than it should be, or if a modulewithin the power converter 22 develops a fault.

The degrade mode manager 56 therefore relates to the level of power thatthe wind turbine generator 1 is able to produce at a fundamental level,in view of either safety considerations or physical constraints.

To this end, the degrade mode manager 56 is also arranged to degrade thepower generating capability of the generator 20 according to thetemperature of at least one component of the wind power plant 12. Forexample, the power generating capability can be degraded if it isdetermined that the temperature of a component is close to or exceeds apredetermined threshold of the component, which could be an operatingtemperature limit or an overall temperature limit of the component.Temperatures exceeding the former limit could compromise the performanceof the component, whereas temperatures exceeding the latter limit wouldput the component at risk of developing a fault and becoming a firehazard. This means that the wind turbine generator 1 would need to beshut down in order for the component to be repaired or otherwisereplaced, resulting in lost production time.

Some examples of components whose temperatures can be used to degradethe power generating capability of the generator 20 are as follows: theinductors 38; the AC-DC converter 26; the DC-AC converter 28; the switch31; the resistor 32; a stator breaker (not shown) used in the generator20; and, the respective circuit breakers (not shown) used in power lines16. However, it will be apparent to those skilled in the art that thislist of components is not exhaustive and that the temperature of othercomponents of the wind power plant 12 could be used when determiningwhether to degrade the power generating capability of the generator 20.

The thermal capability manager 49 is arranged to calculate thetemperature of components of a power converter and output a deratereference to the power management module 36 based on comparison betweenthe calculated temperature and a predetermined threshold of the powerconverter or a component forming part of a power converter. Theoperation of thermal capability manager 49 is described below in moredetail with a focus on a single component of the wind power plant 12. Itwill be apparent to the skilled person, however, that the thermalcapability manager 49 is capable of calculating and carrying out acomparison of temperatures for multiple power converter or componentsforming part of a power converter simultaneously.

FIG. 4 shows the architecture of the thermal capability manager 49 inschematic form. The thermal capability manager 49 includes a series ofmodules that each provides a dedicated function, which process sampledata received from the sampling system in a sequence of stages for thepurpose of generating derate references that are passed to the powermanagement module 52. The derate references limit the output of thegenerator 20 and the converter 22 in accordance with the estimatedtemperatures of power converter or a components forming part of a powerconverter of the wind power plant 12. It will be appreciated by thoseskilled in the art that the derate references can also be generatedbased on direct measurements of a component's temperature whereavailable:

Specifically, the thermal capability manager 49 comprises four modules:an operating point perturber 60; a component thermal module 62; aconverter capability evaluator 64; and, a derate controller 66. Thesemodules may be embodied as software blocks, for example, oralternatively as dedicated hardware components. Although only onecomponent thermal model 62 is shown for simplicity, in practice arespective component thermal model is included for each component thatis to be monitored:

The operating point perturber 60 receives input data including thepresent active and reactive power references, and sample data indicatingoperating parameters such as voltage and current at various stages ofthe wind power plant 12, as well as ambient temperature.

With reference to FIG. 5, the component thermal model 62 comprises threesoftware blocks: a component circuit model 67; a component loss model68; and, a thermal model 70.

The component circuit model 67 simulates the voltage across and thecurrent thorough the component based on the input data received from theoperating point perturber 60 relating to the present active and reactivepower references.

Once the voltage and current have been determined, they are in turn usedas inputs for a component loss model 68, which is configured todetermine the associated power dissipated by the component. Thedissipated power is then used, together with an ambient temperature ofthe component, as inputs for the thermal model 70, which estimates thecurrent temperature of the component.

The ambient temperature in this instance is a measurable temperaturenearest to the component. That is, the ambient temperature is anindication of the temperature of an area surrounding the component sofar as it is reasonably practicable to measure. This may include, forexample, the air temperature or the temperature of the cooling system inproximity of the component. The ambient temperature can be eitherestimated or measured using conventional methods or systems, all ofwhich will be familiar to the skilled reader.

FIG. 6 shows an example of a thermal model, generally designated by 70,for use in the component thermal model 62. The thermal model 70 is ahigh-order model comprising a plurality of Foster circuits 72 arrangedin series. However, it will be apparent to the skilled reader than thethermal model 70 could also be implemented as a first, second, orhigher-order model. In this example, the same heat flow, which isequivalent to the power loss determined using component loss model,passes through each Foster circuit 72. However, it will be apparent tothe skilled reader that a component might be exposed to more than oneheat flow where the same thermal model is shared between components.Each Foster circuit 72 comprises a capacitor 74 and a resistor 76arranged in parallel, and the sum of the voltage drops across each partequals the temperature difference between the component temperature andthe ambient temperature. This can be expressed simply as:

$T_{component} = {T_{amb} + {\sum\limits_{i = 1}^{n}{\Delta \; T_{i}}}}$

Each Foster circuit 72 is a first-order function, which can be describedby a time constant and a gain (H):

${H(s)}\overset{\Delta}{=}{\frac{\sum_{i = 1}^{n}{\Delta \; {T_{i}(s)}}}{P_{loss}(s)} = {\sum\limits_{i = 1}^{n}{R_{i}\frac{1}{{\tau_{1}s} + 1}}}}$

Where,

τ_(i) =R _(i) ·C _(i)

The component temperature can then be estimated as:

T _(component)(s)=H(s)P _(loss)(s)+T _(amb)(s)

The output from the component thermal model 62 is passed to theconverter capability evaluator 64, which compares the temperature of thecomponent against a predetermined threshold.

The converter capability evaluator 64 outputs an indication as towhether the estimated temperature of the component substantially equalsor exceeds the predetermined threshold to the derate controller 66. Ifit is determined that the estimated temperature of the componentsubstantially equals or exceeds the predetermined threshold, the deratecontroller 66 generates respective derate factors for active power andreactive power accordingly. The derate factors are passed to the degrademode manager 56 of the power management module 50, which uses them toupdate active and reactive power references that are used to controloperation of the converter 22 and the generator 20. In the event thetemperature of the component exceeds the predetermined threshold, thederate factors may be proportional to or non-linear with respect to theextent to which the threshold is surpassed.

The degrade mode manager 56 calculates degrade factors of between 0 and1 that are applied globally throughout the system. In a simplifiedexample, if the degrade mode manager 56 determines that the generator 20is only capable of outputting half of its normal capacity in terms ofactive power due to an excessive component temperature, the degrade modemanager 56 calculates a degrade factor of 0.5 for active power.

In this respect, it is noted that as excessive component temperaturesshould be avoided, in this embodiment the derate factors that arecalculated by the derate controller 66 define a maximum capability ofthe converter 22 and, in turn, the generator 20. Any other factors thatmight prompt further degradation, such as those mentioned below, arethen applied within the parameters established by the derate factors.So, for example, if the derate factors are set at 0.8, the degrade modemanager will output a degrade factor in the range 0 to 0.8.

The degrade factors calculated by the degrade mode manager 56 are outputto the power capability manager 54, which uses the factors to update aP-Q chart that defines the ratio of active power to reactive power thatthe wind turbine generator 1 is able to produce, as well as absolutemagnitudes for each type of power. An example of a P-Q chart 80 that maybe used by the converter controller 36 is shown in FIG. 7, which plotsactive power in kilowatts, on the x-axis, against reactive power inkilovolt-amperes reactive, on the y-axis.

A solid line 82 forming a trapezoidal shape represents the capability ofthe generator 20 when operating at normal capacity. The skilled readerwill appreciate that this shape is typical for any P-Q chart for agenerator 20 of a wind turbine generator. Within the solid line 82, adashed line 84 forming a smaller trapezium represents a degradedcapability for the generator 20.

It is noted that in the example shown in FIG. 7, both active andreactive power are degraded in the degraded capability represented bythe dashed line 84, and each by equal amounts. However, in otheroperating modes only one of these may be degraded, or differentweighting may be applied to each type of power. For example, if activepower is prioritised over reactive power, reactive power is degraded toa greater extent than active power, and optionally only reactive poweris degraded. Correspondingly, if reactive power is prioritised, activepower is degraded to a greater extent than reactive power.

The lines shown on the P-Q chart 80 therefore define the long-term powergenerating capability of the wind turbine generator 1. The powercapability manager 54 updates the P-Q chart 80 according to the degradefactors generated by the degrade mode manager 56, if those factors fallbelow 1.

The power capability manager 54 then generates active and reactive powerlimits by checking the updated P-Q chart 80 against a prioritisation ofactive power against reactive power, which is defined by an operatingmode of the wind turbine generator 1 as indicated by the power plantcontroller or the turbine controller.

For example, if reactive power is prioritised, but a reactive powerreference supplied by the power plant controller or the turbinecontroller cannot be met within the limits of the updated P-Q chart 80,the power capability manager 54 adjusts the active and reactive powerlimits accordingly by degrading the active power limit further to enablethe reactive power demand to be met.

In turn, once the power capability manager 54 has updated the P-Q chart80 in accordance with the degrade factor supplied by the degrade modemanager 56, and generated active and reactive power limits in accordancewith the prioritisation between the two types of power, those powerlimits are communicated back to the power plant controller or turbinecontroller as a request for power reduction. The power plant controllerand turbine controller can then take the request into account whengenerating the next set of power references, thereby providing afeedback loop for this element of the control. In this way, the changesdefined by the derate factors based on the temperature of componentscompared with their respective predetermined threshold are propagatedthroughout the wind power plant 12.

The updated P-Q chart 80 is transmitted to the power reference manager52, which also receives several power references from various sources.In this respect, the power reference manager 52 includes an input (notshown) that is configured to receive the various power references. Thepower reference manager 52 further includes a processor 58 that isarranged to analyse the input power references to determine outputactive and reactive power references, and an output (not shown)configured to transmit those references to the power converter 22, asshall be described.

The references received at the input of the power reference manager 52include the active and reactive power references received from the powerplant controller or turbine controller, along with various internalactive power references that together define the auxiliary demand.

The power reference manager 52 also prioritises reactive power overactive power or vice-versa in the short-term according to the sameprioritisation applied by the power capability manager 54. This entailssetting an active or reactive power reference that is outside of the P-Qchart 80 for a short period to meet the prioritised type of demand, asexplained in more detail below.

Typically, the power plant controller or turbine controller issues anactive power reference indicating the level of real power that the windpower plant 12 must deliver, along with a reactive power reference. Asan alternative to a reactive power reference, or in addition to one, thepower plant controller or turbine controller may supply a power factor(or ‘CosPhi’) reference, that defines the ratio of real power to thetotal power dissipated in the system, or ‘apparent power’, in which casethe power reference manager 52 is responsible for determining a reactivepower reference based on the active power reference and the power factorreference. As the skilled person would understand, the reactive powerreference can be derived from these inputs using basic geometric andtrigonometric relations. The power reference manager 52 may have theoption either to calculate the reactive power reference from the powerfactor and active power references, or to use the reactive powerreference supplied by the power plant controller or turbine controller.

The power reference manager 52 compares the power references that itreceives with the present capability of the wind turbine generator 1 asindicated by the P-Q chart 80 received from the power capability manager54, and determines whether the demands to which those references relatecan all be met whilst simultaneously supplying adequate reactive power.

If the demands can be met, the power reference manager 52 simplygenerates active and reactive power references that represent therespective totals of the different active and reactive power referencesthat it receives. If demand cannot be met within the constraints of theP-Q chart 80, the power reference manager 52 prioritises the referencesthat it receives according to a pre-determined regime.

By creating the active and reactive power references based on thevarious demands arising throughout the system, the power referencemanager 52 avoids operating the wind turbine generator 1 at itsoperational capacity—as indicated by the power capability manager 54—atall times. This in turn increases operational efficiency and reduces therisk of failures due to excessive component temperatures.

In summary, the present invention makes use of the functionality of thethermal capability manager 49 to monitor the temperature of componentswithin the wind power plant 12. This in turn is used to update deratefactors that are transmitted to the power management module 52, whichare incorporated into degrade factors that define limits on thecapability of the converter 22 as defined by the P-Q chart 80. Bymonitoring several components, the capability manager 49 provides anoverall derate for the system of components that are evaluated. Thiseventually feeds through to the final active and reactive powerreferences that are output by the power reference manager 52 andtransferred to the active power controller 46 and reactive powercontroller 48 accordingly. This in turn feeds back to the turbinecontroller and thus effects limitation of the generator 20 output asrequired.

The thermal capability manager 49 of this embodiment can also be usedfor modelling the effect over time on the power converter or a componentforming part of a power converter if the wind turbine generator 1 isoperated up to operational limit. In this case, the power converter or acomponent forming part of a power converter thermal model 62 is used forthe purpose of determining the temperature evolution of a componentstarting from its initial temperature based on notional or perturbedpower references. The notional power references define a short-termlimit for apparent power produced by the generator 20 outside the normaloperating limits defined by the P-Q chart 80. The short-term limit isrepresented by dashed line 86 in FIG. 7 and relates to operating thewind turbine generator 1 at its operational limit. It can be seen fromthis figure that the dashed line 86 forms a trapezoidal shape. However,it will be apparent to the skilled reader that the dashed line 86,defining the short-term limit of the wind turbine generator 1, couldalso form different shapes. Within the apparent power limit, thereactive and active power components can be varied to suit instantaneouspriorities. In this context, ‘short-term’ typically entails a durationof a few minutes at most. Further notional power references, defining amedium-term limit, lying between the normal operating limit 82 and theshort-term limit 86, may also be considered but is not presented here.Moreover, numerous short and/or medium-term limits can be continuouslyevaluated.

The short-term limit is determined according to the duration over whichthe wind turbine generator 1 can be operated at its operational limitbefore the temperature of a power converter or a component forming partof a power converter reaches its predetermined threshold.

To this end, the operating point perturber 60 receives input dataincluding the present active and reactive power references, the notionalpower references defining the operational limit of the wind turbinegenerator 1, and sample data indicating operating parameters such asvoltage and current at various stages of the wind power plant 12, aswell as the ambient temperature and the temperature of the component.The notional power references can either be fixed or commanded/adjustedonline.

The component circuit model 67 simulates a set of voltages across andcurrents thorough the component based on the input data received fromthe operating point perturber 60 relating to the present powerreferences and the notional power references.

Once the voltages and currents have been determined, they are in turnused as inputs for the component loss model 68, which is configured todetermine the associated power dissipated by the component. Thedissipated power is then used, together with an ambient temperature ofthe component, as inputs for the thermal model 70, which determines thetemperature evolution of the component at the current power referencesand at the notional power references.

FIG. 8 shows two examples of the temperature evolution of a component,one example based on current power references, indicted by line 90, andanother example based on notional power references, represented by line92. Horizontal line 93 indicates a predefined threshold of thecomponent. In this embodiment, the predefined threshold is theoperational temperature of the component, but it could also relate tothe component temperature limit. It can be seen from this figure thatthe wind turbine generator 1 could be operated at the current powerreferences indefinitely without the temperature of the componentreaching its operational temperature limit. However, line 92 indicatesthat the wind turbine generator 1 could only operate at the notionalpower references for a finite period, denoted by T_(LIMIT), before thetemperature of the component reaches its limit or thermal capacity. Inthis instance, T_(LIMIT) corresponds to the short-term limit forapparent power produced by the generator 20 outside the normal operatinglimits defined by the P-Q chart 80. That is, T_(LIMIT) corresponds toshort-term limit for operating the generator 20 at the inputted notionalpower references. Several short-term limits can be calculated fordifferent notional power references.

The converter capability evaluator 64 calculates T_(LIMIT), which isthen passed to the power management module 50. The power managementmodule 50 then uses T_(LIMIT) to update active and reactive powerreferences that are used to control operation of the converter 22 andthe generator 20 as described above. That is, T_(LIMIT) is communicatedto the power plant controller or the wind turbine generator controllerin order to establish the thermal capacity of the component.

For example, if the power reference manager 52 determines that theessential services cannot be sustained whilst also supplying some powerto the grid, it can provide short-term boosting by prioritising activepower over reactive power. In other words, if the active power demandsof the essential services and the grid exceed the active power that theP-Q chart 80 indicates can be supplied, the power reference manager 52increases the active power reference beyond the normal P-Q chart limit,and decreases the reactive power reference accordingly within theconstraints of the total electrical power that the generator 20 canproduce.

In another scenario, if reactive power is prioritised by the power plantcontroller or turbine controller for enhanced stability, the powerreference manager 52 increases the reactive power reference outside ofthe P-Q chart 80 for a short time, within the boundaries defined by theshort-term apparent power limit 86.

This illustrates that the power reference manager 52 provides a boostingfunction to enable active and/or reactive power to be supplied to thegrid in the short-term, for example when local wind speed is low and sothe generating capability of the wind turbine generator 1 is curtailed.In this way, the power reference manager 52 makes it possible for thewind turbine generator 1 to operate at its maximum generating potentialfor short periods in a safe manner.

As mentioned above, numerous short and/or medium-term limits can becontinuously evaluated. This means that the calculation of T_(LIMIT) isiterative and is updated according to information from the previousiteration. Thus, if the short-term thermal capacity of a component hasbeen used, then the T_(LIMIT) will decrease to zero. T_(LIMIT) will thenincrease while the component cools. The decrease of T_(LIMIT) isactually what allows for short term operation outside normal F-Q chart80 while still ensuring that the component does not reach or exceed itsoperational limit.

In the long-term, the normal or derated limits 80, 84 determined by thepower capability manager 54 must be adhered to so that the wind turbinegenerator 1 does not operate outside of its intended range for longperiods, to avoid prolonged thermal stress within the wind turbinegenerator 1, which could lead to wear or failure as mentioned above. So,whenever the power reference manager 52 determines that the demands foractive and reactive power made by the power plant controller or theturbine controller can be met whilst operating within the normal P-Qchart 80 defined by the power capability manager 54 and whilst alsosatisfying the present prioritisation between active and reactive power,the power reference manager 52 calculates active and reactive powerreferences for the respective controllers that fall within those limits.If power cannot be supplied to the grid without operating outside theP-Q chart 80 for a longer period, the wind power plant 12 must be shutdown until generating capacity become sufficient for stable operation.

As already noted, embodiments of invention are also applicable to othertypes of wind turbine system, including DFIG topologies having a doublyfed induction generator with a rotor-connected converter. Although theskilled person will be familiar with such arrangements, for completenessFIG. 9 shows in overview an example of a wind power plant 81 having suchan architecture.

The wind power plant 81 of FIG. 9 has a generator 83 comprising a set ofrotor windings that are driven by the rotor 2, and a set of statorwindings. To enable the generator 83 to produce electrical power whenthe rotor windings rotate, an excitation current is fed to the rotorwindings by a power converter 85.

The output of the generator 83 is connected to a three-way couplingtransformer 87 that provides electrical connection to a point of commoncoupling (not shown) to a grid, and to the power converter 85. In turn,the power converter 85 is connected to the rotor windings of thegenerator 83, thereby defining a feedback loop. Thus, once powergeneration commences, the power converter 85 can use the output of thegenerator 83 to produce the excitation current that is delivered to therotor windings.

In summary, the power management module 50 of this embodiment separatesthe power management of the wind turbine generator 1 into two distinctcategories, namely long-term power management for stable operation, andshort-term power management for temporary boosting of active powerand/or reactive power when required.

The skilled person will appreciate that modifications may be made to thespecific embodiments described above without departing from theinventive concept as defined by the claims.

1. A method for use in controlling a wind turbine generator of a windpower plant based on a condition of a power converter or a componentforming part of a power converter in the wind power plant, the methodcomprising: determining an initial condition of the component;determining an evolution of the condition from the initial conditionbased on notional power reference values of the wind turbine generator;comparing the evolution of the condition to a predetermined threshold;and, determining, from the comparison, a time period by which thecondition of the component will substantially equal the predeterminedthreshold.
 2. The method of claim 1, further comprising determining,based on the power reference values, an active power limit and areactive power limit.
 3. The method of claim 2, further comprisingprioritising prioritizing between active power and reactive power whendetermining the active power limit and the reactive power limit.
 4. Themethod of claim 1, further comprising controlling the wind turbinegenerator at the notional power reference values for a duration notexceeding the time period.
 5. The method of claim 1, wherein thecondition of the component is the temperature of the component.
 6. Themethod of claim 5, wherein the initial temperature is determined as anon-linear function based on the power loss of the component at currentoperating parameters and an ambient temperature.
 7. The method of claim5 or 6, wherein the temperature evolution is determined as a non-linearfunction based on the power loss of the component at the notional powerreference values and an ambient temperature.
 8. The method of claim 6 or7, wherein the power loss of the component is determined based on thevoltage across the component and the current through the component. 9.The method of claim 1 when applied to a plurality of components, furthercomprising selecting, from a plurality of time periods, the shortesttime period as the time period.
 10. The method of claim 1, furthercomprising: comparing the initial condition of the component to thepredetermined threshold; and, modifying the power reference values ofthe wind turbine generator if the condition substantially equals orexceeds the predetermined threshold.
 11. (canceled)
 12. (canceled) 13.(canceled)
 14. A controller for a wind turbine generator, comprising:one or more processors; and a memory module communicatively connected tothe one or more processors, wherein the memory module comprises a set ofprogram code instructions which when executed by the one or moreprocessors implement an operation for controlling the wind turbinegenerator based on a condition of a power converter or a componentforming part of a power converter in the wind power plant, the operationcomprising: determining an initial condition of the component;determining an evolution of the condition from the initial conditionbased on notional power reference values of the wind turbine generator;comparing the evolution of the condition to a predetermined threshold;and determining, from the comparison, a time period by which thecondition of the component will substantially equal the predeterminedthreshold.
 15. A wind turbine, comprising: a tower; a nacelle disposedon the tower; a generator disposed in the nacelle; a controllerconfigured to perform an operation for controlling the generator basedon a condition of a power converter or a component forming part of apower converter in a wind power plant, the operation comprising:determining an initial condition of the component; determining anevolution of the condition from the initial condition based on notionalpower reference values of the wind turbine generator; comparing theevolution of the condition to a predetermined threshold; anddetermining, from the comparison, a time period by which the conditionof the component will substantially equal the predetermined threshold.